The immune system of pregnant women is tightly controlled to defend against microbial infections and at the same time, to accept an embryo or the fetus, which are expressing semi-allogenic paternal antigens. Furthermore, inflammation-like processes are crucial for tissue growth, remodeling, and differentiation of the decidua during pregnancy. Dysregulation of elaborate immune control may lead reproductive failure, such as implantation failure, recurrent pregnancy loss (RPL), preterm birth, intrauterine fetal growth restriction, and preeclampsia. Until recent years, a balance between Th1 and Th2 cells was believed to be the key immune regulatory mechanism of T-cell immunology especially during pregnancy. Since the identification of regulatory T cells was made, the mechanism of immune regulation has become a major issue in immunologic research. Also, the recent identification of Th17 cells has drawn our attention to a new immune effector. The balance between Th17 and regulatory T cells may explain more about the pathophysiology of reproductive failure. This review will discuss relevant human literature on regulatory T and Th17 cells in normal reproductive physiology and in women with RPL and infertility.